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Exploring Coprime Roots in Quadratic Equations (Posted on 2024-04-09) Difficulty: 3 of 5
For how many natural numbers n<452 are there coprime natural numbers a and b such that 45-√n is the root of the equation x2-ax+b=0?

No Solution Yet Submitted by Danish Ahmed Khan    
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Some Thoughts a solution if double root | Comment 4 of 7 |
What if both roots are the same:   45 - √n
Using Vieta's formulas:
Sum of roots is a = 90 - 2√n
prod of roots is b = 45^2 - 90√n + n

Since a is positive integer, n must be a square in {0^2, 1^2, 2^2, ..., 44^2}
Thus there are only 45 choices for n, and only 45 choices for a.
a can be in {90, 88, 86, ..., 2}

I put these into a spreadsheet and found one solution where a and b are coprime:

(a,b,n) = (2,1,44^2)
x^2 - 2x + 1
root is 45 - √(44^2) = 1

So one value of n is 44^2 = 1936

https://www.desmos.com/calculator/tu4h87cr7h

I have not fully worked out the case where 45 - √n is one of the roots and the other root is something else.

Edited on April 10, 2024, 10:43 am
  Posted by Larry on 2024-04-10 10:41:14

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